The invention relates to a method and system which combines decision feedback equalization and forward error correction to improve performance in point-to-multipoint digital transmission systems.
Many channels used for digital data transmission subject the received signal to a variety of sources of degradation, including Guassian noise, time dispersion, and impulse noise. A twisted pair used for digital subscriber loops is a particularly severe example of such a channel, where non-stationary noise sources such as narrowband interference (induced by AM radio, amateur radio) and periodic impulse noise (e.g., due to light dimmers) can also be a problem. To achieve the desired performance, powerful signal processing techniques are required to recover the data. Two particular techniques that have found widespread use is decision feedback equalization and forward error correction (FEC).
A decision feedback equalizer (DFE) is a non-linear equalizer that uses decisions on previously transmitted data symbols to suppress the intersymbol interference (ISI) due to past data symbols. The structure of a DFE 10 is shown in FIG. 1, where a discrete-time, baseband equivalent model of the channel is assumed. The DFE includes a forward filter 12, a signal adder/subtractor 14, a decision element 16, and a feedback filter 18. A sample at the input to the forward filter contains contributions due to both past and future symbols. The forward filter 12 serves to at least suppress the contribution of future symbols as well as those of past symbols outside the span of the feedback filter. The feedback filter 18 then uses decisions on previous symbols to generate an estimate of the remaining ISI, which is subtracted from the decision variable. If the filters are sufficiently long and the previous decisions are correct, the DFE can remove ISI without the significant noise enhancement that can result if the feedback filter is not included. While not included in the foregoing description, it will be appreciated that DFEs can be either realvalued or complex-valued and the forward filter can perform other functions, such as frequency translation.
A drawback to decision feedback equalization is that decision errors within the span of the feedback filter will degrade the estimate of the ISI due to past symbols. The resulting decision variable will thus include residual ISI as well as noise, increasing the probability that the current decision will be incorrect. If this effect leads to further errors, the phenomenon is known as error propagation. The extent of error propagation is a function of the magnitude of the residual ISI, which in turn is a function of the size of the data alphabet and the magnitude of the coefficients of the feedback filter. When the channel is severely distorting, and especially when strong narrowband interference is present, the feedback filter taps can become quite large. Large signal alphabets are also quite common for high rate transmission, such as over twisted pair for digital subscriber loops. A DFE used in these environments can thus be subjected to a situation where error propagation can continue for an unacceptably long time. This is known as catastrophic error propagation.
In point-to-point transmission, error propagation can be avoided through the use of preceding. This technique entails performing the feedback portion of the DFE operation at the transmitter, where the data is known. To do this, the transmitter must implicitly have knowledge of the channel response. In a point-to-multipoint environment, however, the transmitted signal traverses multiple channels, and it is not possible to use the preceding method. Conventionally, in this situation, the assumption has been that the effects of error propagation can be accepted. However, for high-speed transmission such as the digital subscriber line, it must be assumed that error propagation will be significant and will continue until a successive number of correct decisions are made at least equal to the length of the feedback filter.